POSTNATAL-DEVELOPMENT OF CA3 PYRAMIDAL NEURONS AND THEIR AFFERENTS INTHE AMMONS HORN OF RHESUS-MONKEYS

Previous studies described the postnatal development of CA3 pyramidal neurons and their afferents in the rat. However, the post-natal develo pment of the primate hippocampus was not previously studied. Thus, pyr amidal neurons of the CA3 area of the monkey hippocampus were analyzed postnatally in the present study. At birth, a few thorny excrescences , the complex spines postsynaptic to messy fibers, were found on the p roximal segments of both apical and basal dendrites, whereas distal de ndrites displayed pedunculate spines. Thorny excrescences increased in number until the third month. A continuous increase in the number of spines per unit length along the distal dendrites was observed during the first 12 months. The ultrastructural features of somata and dendri tes of pyramidal cells in newborn monkeys were similar to those of adu lts. The analysis of the afferents to the CA3 pyramidal neurons was li mited to the development of messy fibers, the axons of granule cells, and myelinated axons in the alveus, stratum oriens, and stratum lacuno sum-moleculare. At birth, most messy fiber terminals were densely pack ed with synaptic vesicles and formed mainly axospinous synapses with C A3 pyramidal cells. By 1 month of age, the number of mitochondria and embedded spines increased to mature amounts. In the first postnatal mo nth, degenerating axons and axon terminals were frequently observed in the messy fiber bundles in stratum lucidum. The proportion of myelina ted axons increased simultaneously in all three examined layers. At bi rth most axons were unmyelinated, whereas at 7 months of age the propo rtion of myelinated axons was similar to that found in adults. The pre sent study indicates that most pyramidal neurons of the CA3 region in monkeys are in an advanced stage of development at the time of birth. Thus, messy fibers from granule cells in the dentate gyrus have establ ished mature-looking synapses, and the thorny excrescences of pyramida l cells that are postsynaptic to messy fibers are also adult-like. Nev ertheless, several of the adult features, such as the spine density of distal dendrites of pyramidal neurons and the myelination of afferent axons, develop during an extended period of time in the first year. T he significance of this early anatomical maturation in a brain region involved in memory function is consistent with recent behavioral data that show a rapid postnatal maturation of limbic-dependent recognition memory in rhesus monkeys. (C) 1995 Wiley-Liss, Inc.